Extra stress-mediated acoustic streaming in a surface acoustic wave driven microchannel filled with second-order fluids

This study investigates the acoustic streaming phenomena in a standing surface acoustic wave driven microfluidic channel filled with second-order fluids. We have developed a multiple-timescale-based theoretical model where a perturbation technique was adopted to separate the fast and slow timescales associated with the oscillatory flow field (i.e., acoustic field) and the mean flow field (acoustic streaming), respectively. The governing equations have been expressed in nondimensional form to effectively show the dependence of the acoustic streaming fields on Reynolds number (Re), Deborah number (De), and the ratio of material constants related to normal stress coefficients (b). Contrary to our intuition, we observed that with increasing Deborah number (which is a measure of the extra stress present in the second-order fluids), acoustic streaming first increases and thereafter, a further increase in Deborah number leads to a gradual suppression of streaming. Our study also reveals that the acoustic field and the acoustic radiation force show negligible depen-dence on the fluid rheology. For the following ranges, 0.08 <= Re <= 0.32, 0 <= De <= 1, and 0 <= b <= 100, the maximum variation of the acoustic streaming is observed to be similar to 161.3%, whereas the variation in the acoustic field stays within just 0.15%. This significant finding can help design efficient acoustofluidic systems that can manipulate acoustic streaming without affecting the acoustic radiation forces, as strong acoustic streaming can impair the acoustofluidic devices.

Automated summary

This study investigates the acoustic streaming phenomena in a standing surface acoustic wave driven microfluidic channel filled with second-order fluids. It has been found that increasing Deborah number leads to an initial increase and then gradual suppression of acoustic streaming. The study also reveals that acoustic streaming has negligible dependence on fluid rheology.